1. Field of the Invention
The present invention relates to glow plug control circuits. More specifically, the present invention relates to a glow plug control circuit which provides for the rapid heating of a bank of parallel connected glow plugs to their set temperatures without overheating and for a subsequent temperature controlled intermittent "afterglow" energization of the glow plugs for an ambient-temperature-related time period.
2. Description of the Prior Art
Heretofore, various glow plug control circuits have been proposed for rapidly heating one or more "rapid warmup" glow plugs under controlled conditions to prevent overheating and damage to the glow plugs. One of the first such circuits was developed by the Automotive Products Division of Allied Chemical Corporation and is described in SAE Technical Paper No. 709208 by Arthur R. Sundeen, published Feb. 26, 1979. Such circuit utilizes a bimetal control element which allows for a rapid heating of the glow plugs to a set temperature followed by intermittent heating which can be represented by an undulating waveform as the bimetal element opens and closes electrical circuits as it heats and cools in an "afterglow" delayed energization period.
The controller utilized in the Allied Chemical Corporation control circuit utilizes a timer in which time is based on the characteristics of the bimetal element which serves as a "surrogate" for the glow plugs.
Examples of various glow plug control systems that use resistance bridges, comparators, control circuits and/or timer circuits are disclosed in the following patents:
______________________________________ PATENTEE ______________________________________ U.S. Pat. No. 3,803,385 Sandorf 3,866,587 Knapp 3,955,067 Eldridge 4,164,261 Igashira et al. 4,196,712 Kawamura et al. 4,280,452 Kawamura et al. 4,375,205 Green 4,377,138 Mitani et al 4,391,237 Abe et al. 4,399,781 Tsukasaki 4,444,160 Steele 4,445,491 Ishikawa et al. 4,452,191 Steele 4,458,639 Abe et al. 4,483,284 Andreasson 4,491,100 Yasuhara 4,493,298 Kawamura U.K. Patent Application Kawamura et al No. 2 024 940 Japanese Published Morino Patent Application No. 54-5137 ______________________________________
As will be described in greater detail hereinafter, the glow plug control circuit of the present invention differs from the previously proposed circuits by providing a compact, integrated control circuit unit which can be mounted directly on a diesel engine but which is not limited to engine mounting. The circuit is able to take advantage of positive temperature coefficient (PTC) glow plug characteristics automatically to control the temperature of a variety of such glow plugs thereby providing a glow plug heating system which is insensitive to system voltage or fluctuations thereof and to ambient temperature. Heating and control thereof is achieved by utilizing a bridge circuit coupled to a comparator which, in conjunction with a low frequency oscillator clock, control the outputs of a bi-stable multivibrator which controls the energization of the glow plugs.